In the field of liquid crystal display, the active layer of thin film transistor (TFT) often uses silicon-based materials which have a good stability and processability. Silicon-based materials are mainly divided into two categories: amorphous silicon and polycrystalline silicon. Amorphous silicon offers very low carrier mobility. While polycrystalline silicon offers higher mobility, polycrystalline silicon-based devices often have poor uniformity, low yield and high unit price. Thus, technologies of manufacturing transparent metal-oxide semiconductors as the active layer of TFTs (i.e., metal-oxide TFTs), which are adopted in electronic devices and optical devices, have attracted widespread attention recently. Field-effect amorphous indium gallium zinc oxide (a-IGZO) TFTs, have been widely used because of their higher carrier mobility, higher on-to-off current ratio, better large-size uniformity, and lower processing temperature, etc.
Metal-oxide TFTs are often divided into two categories: bottom-gate TFTs and top-gate TFTs. A bottom-gate metal-oxide TFT includes a substrate, in which a gate, a gate insulating layer, a metal-oxide semiconductor layer, a source/drain conductive layer, and a protective layer are sequentially disposed. A portion of the metal-oxide semiconductor layer between the source and the drain defines a channel or channel region or channel layer.
In practical applications, the metal-oxide TFTs are inevitably irradiated by ambient light. Thus, due to the nature of the metal-oxide semiconductor layer, such light irradiation can cause light-induced defects in the metal-oxide semiconductor layer. When an external voltage is applied to the metal-oxide TFT, these light-induced defects diffuse to an interface between the channel and the gate insulating layer, where an interfacial state (i.e., the semiconductor interface is located in energy levels of a forbidden band and quickly exchange charges with the semiconductor) can be formed. The interfacial state can result a shift in the threshold voltage (Vth) of the metal-oxide TFT, i.e., voltage necessary to turn on the metal-oxide TFT, and accordingly affect the stability of the metal-oxide TFT.
To eliminate the light-induced defects in the metal-oxide TFTs under light irradiation, currently two methods are often adopted.
The first method is to grow a protective layer (e.g., an ESL or a passivation layer) on the channel layer, such as SiO2, Al2O3, Y2O3, etc., to increase the barrier for generating light-induced defects in the back channel region under light irradiation, and accordingly reduce the influence of the light irradiation on the metal-oxide TFT performance. However, if the light irradiation is strong, the light irradiation is still able to penetrate the protective layer on the back channel region and the performance of the metal-oxide TFT is still affected.
The second method is to adopt an opaque metal layer or multiple layers as a light-shielding layer to cover the semiconductor channel region and to eliminate the influence of the light irradiation on the stability of the semiconductor device (i.e., the metal-oxide TFT). However, the fabrication of the light-shielding layer requires an additional photomask and, thus, the fabrication cost and fabrication complexity are increased accordingly.
The disclosed array substrate and fabrication method thereof, and the corresponding touch screen and touch display device are directed to solve one or more problems in the art.